1. Field of the Invention
This invention relates to a spark plug for an engine for a cogeneration system.
2. Description of the Prior Art
In the gas engine in a cogeneration system, fuel is ignited with a spark plug. In this spark plug, the heat load to the electrode is greater than that to the spark plug for automobile engines because the engine for the cogeneration system is required to continuously run.
U.S. Pat. No. 5,369,328 discloses a spark plug for cogeneration comprising ground electrodes confronting each other, and a central electrode arranged between the ground electrodes. The central electrode has a prism-shaped attachment portion which is square in plan view and which comprises a platinum-rhodium alloy. The prism-shaped attachment portions carried by a pin-shaped part of the center electrode which projects into a central opening in the attachment portion and is welded or soldered or brazed thereto at the top surface of the prism-shaped portion. As a result, the weld portion is subject to affection of the heat from the combustion gas. Therefore, the weld portion that is derived from melted platinum-rhodium alloy and the material of the central electrode cannot resist the heat load for long interval, so that there is considerable wear at that portion.
The aim of the present invention is to provide a superior spark plug.
According to the present invention, a first aspect of the present invention provides a spark plug for a cogeneration engine comprising: a tubular housing; a central bar electrode supported by said tubular housing in said tubular housing with electrical insulation therebetween; a first bar discharge member being arranged at one end of said central bar electrode, comprising Ir alloy, and protruding from one end of said tubular housing along an axis of said center electrode; a plate ground electrode being arranged at said one end of said tubular housing in a radial direction of said axis and having one end surface confronting a side surface of said first bar discharge member; and a second discharge member being arranged on said end surface and having a surface confronting said side surface, spark discharge being generated at a gap between said first and second discharge members, wherein if it is assumed that a distance of said gap is G, a width of said side surface in a normal direction of a plane including said radial direction and said axis is D, and a width of said surface in said normal direction is A, G is equal to or greater than 0.2 mm and equal to or lower than 0.6 mm, D is equal to or greater than 1.6 mm, |Axe2x88x92D| is equal to or lower than a result of adding 0.5 mm to G.
According to the present invention, a second aspect of the present invention provides a spark plug based on the first aspect, wherein D is equal to or lower than 5.0 mm.
According to the present invention, a third aspect of the present invention provides a spark plug based on the first aspect further comprises a weld portion between said one end of said central electrode and said first bar discharge member, wherein a maximum cross-sectional area of said weld portion on a plane perpendicular to said axis is equal to or lower than 8 mm2.
According to the present invention, a fourth aspect of the present invention provides a spark plug based on the first aspect, wherein said one end of said central electrode is welded to a surface of said first bar discharge member on the side of said central electrode.
According to the present invention, a fifth aspect of the present invention provides a spark plug based on the first aspect, said spark plug further comprises a weld portion between said one end of said central electrode and said first bar discharge member, said weld portion being provided by laser welding, wherein said weld portion does not confront said surface of said second discharge member, if it is assumed that a shortest distance between said weld portion and said second discharge member is L, L is equal to or greater than G.
According to the present invention, a sixth aspect of the present invention provides a spark plug based on the fifth aspect, wherein L is equal to or greater than a result of adding 0.2 mm to G.
According to the present invention, a seventh aspect of the present invention provides a spark plug based on the first aspect, wherein each of said first and second discharge members comprises Ir and at least one of Rh, Pt, Ru, Pd, and W.